The present invention relates to an extrusion die assembly. Specifically, the present invention relates to an extrusion die assembly comprising a pair of extrusion dies wherein one extrusion die is disposed within the second extrusion die. The extrusion dies may comprise a plurality of extrusion die modules wherein at least one of the extrusion die modules incorporates a flat compression deal with a conical or otherwise angled spillover surface. Further, the present invention relates to a method for utilizing an extrusion die assembly comprising a pair of extrusion dies, each having at least one extrusion die module having a conical or otherwise angled spillover surface.
An extrusion die for extruding melt material for the creation of flexible films has typically included at least one module having one or a plurality of flow channels to distribute melt over a spillover surface. The melt material may generally flow from the flow channels onto the spillover surface to be extruded therefrom to form a flexible film. In a typical flexible film xe2x80x9cbubblexe2x80x9d extruder, the module allows molten material to flow over a spillover surface to a gap between a die module and a central mandrel, whereupon a tube of plastic material may be extruded through an annular portion of the extrusion die. The concept also applies to extrusion blow molding parisons, wire coating, pipe extrusion, tube extrusion, etc. Moreover, a plurality of modules may be stacked in series to extrude a plurality of layers into a tubular structure. Each successive module may add a layer to the film structure when the molten material solidifies.
Extrusion die modules are typically nested cylinders, flat disk, or conical die modules. A flat disk module may allow a molten material to flow through flow channels directly onto a flat spillover surface located adjacent to the central mandrel. Each type of module typically consists of a pair of matching portions, or matching halves, that may be disposed one atop of another to form internal flow channels and spillover surfaces within the module. The matching halves may be bolted together to form a single extrusion die module, whereby the molten material may enter via the flow channels and may be extruded annularly after flowing over the spillover surface.
In melt extrusion, it is often desirable to streamline the flow of material from the flow surface of the module to the flow surface of the annular portion of the extrusion die. For example, the material flow may be streamlined by changing the flow direction angle of the material through the module and over the spillover surface by as small an angle as possible at any given point in the material flow. Flat disk extrusion dies are typically unable to generate a large degree of streamlined material flow because the plastic material generally flows through the flow channels of the module and over the spillover surface until it meets the annular portion of the extrusion die. At the annular portion of the extrusion die, the plastic material flow direction changes approximately 90 degrees to flow along the central mandrel that is located perpendicular to the spillover surface of the die module. Conical extrusion die modules are typically better adapted for promoting streamline material flow. The conical portion allows the flow direction angle to change less than 90 degrees as the material flows over the spillover surface and into the gap between the die module and the mandrel.
Moreover, conical extrusion die modules are capable of having a smaller wetted diameter and a smaller overall diameter than similar flat disk extrusion die modules. For example, the flow path of a conical extrusion die module with a flow path entirely at 45 degrees to the module axis needs only to be 71% (sine(45xc2x0)) of the radial length of a flat disk extrusion die module with the same area of spillover surface. Reducing both the wetted diameter and the overall die diameter has a number of benefits. For example, less wetted diameter means less hydraulic force is exerted by the melt material. Also, less overall module diameter means less module weight to support, move, and otherwise handle. Similarly, less overall module diameter means less thermal mass to heat and cool, thereby allowing for faster startups and changeovers and better thermal control. Each of the above effects may vary proportionally to the diameter of the die module squared (d2) thus enabling a small reduction in diameter to have a significant positive effect.
Leakage of molten material from the modules may cause many problems. For example, the plastic may leak out of the module to the ambient environment, thereby wasting material and creating potential housekeeping and safety issues. Additionally, the plastic material may degrade into carbon, gels, gas, and/or material of altered physical, chemical, and/or optical properties. Backflow of the degraded material may contaminate the extruded product. Further, plastic materials, such as PVdC and PVC, may form corrosive byproducts, if allowed to leak and degrade, and may lead to corrosion of the extrusion die. A damaged extrusion die is even more susceptible to leakage, leading to more degradation, thereby leading to further corrosion. Moreover, plastic flow outside of the flow channel may act as a hydraulic fluid under pressure to exert forces against adjacent extrusion die modules or other equipment. Consequently, the adjacent die modules may separate, causing further leakage and larger separating forces to be generated. Commonly, seal means are used to prevent plastic flow from leaking or otherwise leaving the flow channel areas of the modules.
A number of seal means for extrusion die modules are presently used. For example, flat surface to flat surface compression seals may be utilized for flat disk dies. Moreover, matched taper seals may be utilized for conical dies. Flat surface to flat surface compression seals are generally more effective than matched taper seals because the matched channels that are cut into matching conical die module halves may move out of alignment as the seals undergo surface wear. Surface wear is particularly damaging for conical dies having the flow channels disposed on the spillover surface(s). Because the flow channels are disposed on the angled portion of the spillover surface(s) of the conical die, surface wear causes the flow channels of the matching module portions to fall out of alignment. As the conical die channels move out of alignment, the degradation areas may increase and the problems discussed above may intensify. Conversely, as flat surface compression seal surfaces undergo surface wear, the matched channels that may be cut into the adjacent dies generally maintain their alignment, because the matching module portions are horizontal and the dislocation effect is minimized.
In multi-layer plastic extrusion, it is often desirable to combine certain material layers as early in the coextrusion system as possible. For example, it is desirable to encapsulate degradation-prone layers with more stable layers as soon as possible. Therefore, the more stable layers protect the degradation-prone layers from the heat of the extrusion system and decrease the time that the degradation-prone material is exposed to the heated surfaces of the extrusion system. In a standard flat disk die, the amount of time that a degradation-prone material may be exposed to heat of the annular portion of the die is related to the thickness of the extrusion die module itself.
When flat disk die modules and conical die modules are stacked in series, the distances that the degradation-prone materials are exposed to the surfaces of the extrusion die module is related to the type of die module and order of placement of the die modules in the series. Material layers enter the annular portion of the extrusion die from the extrusion die modules separated by a distance approximately equal to the thickness of one die module, in the case of a first flat disk die module positioned downstream of a second flat disk die module. Similarly, material layers typically enter the annular portion of the extrusion die from the extrusion die modules separated by a distance approximately equal to the thickness of one die module in the case of a first conical die module positioned downstream of a second conical die module. Further, material layers typically enter the annular portion of the extrusion die from the extrusion die modules separated by a distance approximately equal to one half the thickness of a flat disk die module in the case of a flat disk die module positioned downstream of a conical disk die module. Moreover, material layers enter the annular portion of the extrusion die from the extrusion die modules separated by a distance approximately equal to the thickness of a conical die module plus one half the thickness of a flat disk die module in the case of a conical disk die module positioned downstream of a flat disk die module. Therefore, stacking known flat disk and conical dies in series may cause a degradation-prone material to be exposed to the hot and potentially damaging surface of the die modules and/or the mandrel for at least one half of a die module. In many instances, it is beneficial to encapsulate a degradation prone material layer with a shorter distance between the entry of the multiple layers into the annular portion of the extrusion die.
Accordingly, it is desirable to provide an extrusion die module with the flat surface compression sealing benefits of a flat disk extrusion die and the streamlined flow and reduced diameter benefits of a conical extrusion die. Additionally, it would be beneficial to provide a method of combining multiple material layers within a shorter distance along the annular portion of the extrusion die to allow degradation-prone materials to be encapsulated by more stable layers and to minimize the distance that a degradation-prone layer is exposed to direct contact with the walls of the extrusion die.
The present invention relates to an extrusion die assembly. Specifically, the present invention relates to an extrusion die assembly comprising a pair of extrusion dies wherein one extrusion die is disposed within the second extrusion die. The extrusion dies may comprise a plurality of extrusion die modules wherein at least one of the extrusion die modules incorporates a flat compression deal with a cone-shaped spillover surface. Further, the present invention relates to a method for utilizing an extrusion die assembly comprising a pair of extrusion dies, each having at least one extrusion die module having a conical or otherwise angled spillover surface.
It is one of the principal objectives of the present invention to provide an extrusion die assembly having a pair of extrusion dies, wherein one extrusion die is disposed within the other extrusion die. Further, it is an objective to provide a plurality of extrusion die modules within each extrusion die. In addition, it is an objective to provide an extrusion die module having a horizontal flat surface to horizontal flat surface compression seal outside of the distribution channels of the extrusion die module. It is another objective of the present invention to provide an extrusion die module with a conical, arched, or vertically cylindrical spillover surface that can be incorporated into the extrusion die assembly.
It is yet another objective of the present invention to provide an extrusion die module capable of being used to streamline the flow of material being extruded through the die module and through the annular portion of the extrusion die. It is still another objective of the present invention to provide an extrusion die module with a reduced overall diameter.
It is moreover an objective of the present invention to provide an extrusion die module with a reduced wetted diameter. It is a further objective of the present invention to provide an extrusion die module capable of being used to combine layers of material flowing through separate dies modules with a minimum distance between the two entry points of the two layers into the annular portion of the extrusion die.
These and other objectives of the present invention will become apparent upon examining the drawings and figures together with the accompanying written description thereof.